Unvented Roof for Cold Climate (Cathedral Ceiling); Asphalt Roofing

Where Roofs Meet Walls is a Critical Connection

Corners and connections are where insulation and air barriers can have trouble. Compressed or insufficient insulation can cause cold spots, which lead to condensation, mold, and rot. Air leaks at this connection can cut the effectiveness of the insulation substantially. In cold climates, this is where ice dams begin.

To keep the air barrier continuous, span the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. over the framing connection and use adhesive or sealants at framing connections as shown.

Roofs and walls need to dry
Moisture from both outside and inside a house can thwart your best efforts at keeping the building dry. Moisture in roof and wall assemblies is inevitable, so it's a good idea to design them so that they can dry. Roofs and walls that can dry to either the outside or inside are good, but those that can dry both directions are even better.

  1. Designing to dry out means doing two things well:
  2. 1. Choosing materials carefully—each layer affects the vapor profileA vapor profile is an assessment of the relative vapor permeabilities of each individual component in a building assembly and a determination of the assembly's overall drying potential and drying direction based on vapor permeabilities of all of the components. The vapor profile addresses not only how the building's enclosure assembly protects itself from getting wet, but also how it dries when it gets wet. For a detailed treatment of this subject, see Building Science Corporation's article Understanding Vapor Barriers. of the assembly.
  3. 2. Planning the construction to be forgiving—flashing keeps water out, and ventilation removes water vapor.

Unvented roofs can perform well as long as they are properly detailed to limit moisture transfer from the interior. Construction details vary depending on climate, but closed-cell spray polyurethane foam (specifically allowed by Section R806.4 of the International Residential Code) can be used anywhere.

Exterior insulation keeps the framing warm and dry
By moving the insulation outside the framing, the chances of condensation are almost eliminated. Another benefit is that you can get a superinsulated roof without increasing the size of the rafters, or furring the framing down and encroaching on the living space.

For detailed information on this topic, read Unvented Roof Systems at BuildingScience.com

Venting above the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. allows drying
Conditioned attics with unvented roofs can still provide a drying path through ventilation. You can ventilate the roof claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. above the roof sheathing. You can do this the same way you ventilate wall claddings either with a drainage mat or with furring strips positioned above each rafter. A second layer of roof sheathing installed above the furring provides backing the for shingles or shakes.

Learn more in the Green Building Encyclopedia

Enclosure overview
Exterior walls
Roofs: Attics, Structure, Claddings

DRAWING DETAIL

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